A numerical heat-transfer and pressure-drop analysis is presented for porous rings inserted in a pipe at a distance L apart. A constant heat-flux is applied to the outer surface of the pipe. Numerical calculations are conducted with the Fluent 6.1.22 code, using the shear-stress transport (SST) k-omega model. Air is the fluid. The heat-transfer increase is analyzed for Reynolds numbers from 3 x 10(3) to 45 x 10(3). The porous-ring height is taken as H = 1 or 2 mm. The distance between two porous rings is 0.5D, D or 2D where D is the inside diameter of the pipe. An increase in L/D caused a decrease in heat-transfer. High Nusselt numbers were obtained when H/D and L/D ratios were 0.4 and 0.5, respectively, for a Reynold number of 45,000. The maximum Nusselt number occurred when L/D = 1 if H/D is selected as 0.2. (C) 2005 Elsevier Ltd. All rights reserved.